Quotes in the article

Sales of Apple (AAPL)'s flagship product are likely to drop up to 8 percent next year, said Andy Hargreaves and Evan Wingren of technology investment research firm Pacific Crest Securities. The massive number of consumers that upgraded to the iPhone 6 when it was released last September is "unrepeatable," they said in a note Sunday.

The question is: Will existing Apple users be ready to upgrade in the next year?

In a fiscal third-quarter earnings call, CEO Tim Cook said there is "a lot of headroom left for upgraders," as only 27 percent of customers chose to upgrade to an iPhone 6.

But while some have used that rate to justify growth in the coming cycle, Hargreaves and Wingren questioned whether that kind of growth is sustainable.

The speculation that iPhone sales could decline by Hargreaves and Wingren comes at a time when analysts say the smartphone market in general may be near saturation, especially in China.

Apple's fiscal third-quarter sales in China increased 112 percent year over year, according to the company's earnings report, a pace of growth that does not seem sustainable, Hargreaves and Wingren said. While iPhone sales have increased in that Asian nation, Apple has fallen to third place for market share, behind Huawei and Xiaomi, according to research firm Canalys.

"The China smartphone market continues to mature, remaining stagnant quarter on quarter," wrote Canalys analyst Jingwen Wang in a market analysis. "Competition among major brands has never been so intense."

To be sure, Hargreaves and Wingren wrote that the iPhone will remain "the most important device in the world" in 2016. And not all analysts agree that iPhone sales are set to decline.

Toni Sacconaghi and Daniel Chen, analysts at Bernstein, expect iPhone sales to grow 3 percent in 2016. In a note, they pointed to an emerging middle class in China, the opportunity to gain market share from Samsung (593-KR) and more flexible cell phone plans as potential sources of growth.

And a Baird Equity Research survey found strong upgrade demand from current IPhone 6 holders, in a study of 1,500 consumers in the first week of July.

This isn't the first time the too good to be true argument has been made for Apple.

Still, with shares of the company down 11 percent this month, after narrowly beating analyst's earnings expectations for the third quarter of 2015, same-store sales could be a key factor for investors. Sales of iPhones currently represent 63 percent of Apple's quarterly revenue, according to its latest earnings release.

"The iPhone 6 comp is going to be the most important issue going in to this year," Ben Schachter, analyst at Macquarie Research, said Monday on CNBC's "Squawk on the Street." "That's probably going to be what matters most over the next few months. After that is when you start to get an opportunity in the stock."